The detection of biological species within complex systems is important for many applications. Among these many applications, biomedical and environmental applications show perhaps the most immediate benefit, for example, with respect to infectious disease identification, medical diagnostics and therapy, biotechnology, animal husbandry, and genetic plant engineering.
A variety of sensing schemes have been developed for molecular detection, including approaches such as electrochemical, optical absorption, interferometric sensing, and fluorescence sensing. In addition, there are several methods for selectively identifying biological species, including antibody detection and assay (i.e., analysis to detect the presence, absence or quantity of a molecule) using molecular hybridization techniques. Assaying approaches including, e.g., Enzyme-linked Immunosuppresent Assays or ELISA are often used in this manner. Joining a single strand of nucleic acid with a complementary probe sequence is known as hybridization, or incorporation.
Generally, to identify sequence-specific nucleic acid segments, sequences complementary to those segments are designed to create a specific probe for a target cell, such as a pathogen or mutant cell. Nucleic acid strands tend to pair with their complements to form double-stranded structures. Thus, a single-stranded DNA molecule (e.g., a probe), in a complex mixture of DNA containing large numbers of other nucleic acid molecules, will seek out its complement (e.g., a target). In this manner, incorporation provides an accurate way to identify very specific DNA sequences, such as gene sequences from bacteria or viral DNA. Factors impacting the incorporation or re-association of two complementary DNA strands include temperature, contact time, salt concentration, degree of mismatch between the pairs, and the length and concentration of the target and probe sequences.
Pyrosequencing is an assaying approach that is widely used in biotechnology applications for real-time sequencing by synthesis that takes advantage of coupled enzymatic reactions to monitor DNA synthesis. Pyrosequencing offers accurate and consistent analysis of large numbers of short to medium length DNA sequences, and compatibility with standard biochemical reactions, such as polymerase chain reaction (PCR). For example, separation technologies such as capillary array electrophoresis and micro-array technology use pyrosequencing for the detection of analytes such as DNA and proteins. In the presence of a primed DNA template, DNA polymerase incorporates a nucleotide if the nucleotide is complimentary to the target DNA. As a result of incorporation, a pyrophosphate (PPi) molecule is released and consequently detected by a pair of coupled enzymatic reactions, culminating in the generation of light by firefly luciferase. The excess (unreacted) nucleotides are enzymatically removed in real-time, typically using apyrase.
In one approach, pyrosequencing involves first hybridizing a sequencing primer to a single stranded, PCR amplified, DNA template. The DNA template is then incubated with enzymes such as DNA polymerase, ATP sulfurylase, luciferas and apyrase, along with substrates adenosine 5′ phosphosulfate (APS) and luciferin. Thereafter, deoxyribonucleotide triphosphate (dNTP) is added to the reaction. DNA polymerase catalyzes the dNTP into the DNA strand if it is complementary to the base in the template strand, with each incorporation occurrence being accompanied by an equimolar release of PPi to the amount of incorporated nucleotide. In the presence of APS, ATP sulfurylase converts PPi to ATP. In turn, this ATP, mediated by the luciferase, drives the conversion of luciferin to oxyluciferin generating light in the visible spectrum in amounts proportional to the amount of ATP consumed. The characteristics of the light are observable as an indication of the quantity of nucleotides incorporated. Apyrase continuously degrades ATP and any unincorporated dNTP to eventually terminate the light generation and regenerate the reaction so that other dNTPs can be added, one at a time, to the mixture. The complimentary DNA strand is built up, and detected through light indications, as the process continues. Conventional biological assays using approaches including those discussed above, however, are highly repetitive, can be labor intensive and are typically performed with microliter volume samples. The associated biochemical procedures comprising these protocols often require days or weeks to perform at a cost of hundreds of dollars per test. For instance, performing biological assays involving the detection of characteristics of a biologically active substance using an in vivo or in vitro tissue or cell model under controlled conditions can be tedious. Problems remain in reproducibly detecting and measuring low levels of biological compounds conveniently, safely and quickly. Furthermore, there continues to be a need for portable, low-power and low-cost designs that can operate robustly at various temperatures from below zero to room temperature thereby accommodating several different chemistries and applications.